Halogen exchange reactions for fluorinating haloaromatic compounds using alkali metal fluorides have been extensively studied heretofore. Typically they involve the reaction of a chloroaromatic compound with potassium fluoride, rubidium fluoride or cesium fluoride by heating the reactants to extremely high temperatures (above about 400.degree. C.) in the absence of an ancillary diluent or solvent, or by conducting the reaction at temperatures of around 200-230.degree. C. in an aprotic solvent such as sulfolane. It has also been reported that organic fluorine compounds such as pentafluorobenzonitrile, tetrafluorophthalonitriles and pentafluoropyridine can be formed by reacting a corresponding chloro- or bromo-substituted compound with alkali metal halide such as potassium fluoride in benzonitrile as solvent at 190.degree. C. to 400.degree. C. in a sealed autoclave under autogenous pressure.
Use of catalysts in some exchange reactions has also been studied. Such catalysts have included quaternary ammonium salts, metal carbonyls, crown ethers and cryptates.
In most cases, the halogen exchange reaction is sluggish and tends to form product mixtures in which yields of polyfluorinated aromatics are relatively low, especially if the haloaromatic compound used is a polyhaloaromatic compound free from activating functionality such as nitro or carbonyl. For example, with hexachlorobenzene and potassium fluoride, typical product mixtures contain a mixture of co-products including hexafluorobenzene together with various chlorofluorobenzenes.
In now commonly-owned application Ser. No. 08/754,338, filed Nov. 22, 1996, Igor Bildinov et al. describe a significant improvement in halogen exchange technology, namely that aminophosphonium compounds such as one or more tetra(dihydrocarbylamino)phosphonium halides are effective catalysts for halogen exchange reactions. Significant improvements in yields of desired products were achieved by conducting the reaction with a suitably-stirred mixture of reactants and catalyst in the form of finely divided solids.
Tetra(dihydrocarbylamino)phosphonium halides, which are relatively uncommon materials, can be prepared using the method described by Koidan, Marchenko, Kudryavtsev, and Pinchuk, Zh. Obshch. Khim., 1982, 52, 2001, an English language translation of which is available from Plenum Publishing Corporation. A procedure which has been used for preparing tetra(diethylamino)phosphonium bromide involves the following four steps (where Et represents an ethyl group): ##STR1## In this procedure, carbon tetrachloride and phosphorus trichloride are charged to the reactor followed by the slow addition of diethyl amine at low temperature (30.degree. C. maximum). This results in the formation of dichloromethylene phosphoroamidite intermediate. Ammonia (gas) is then charged to the reactor resulting in the formation of an imino hydrochloride phosphoroamidite intermediate. Following a period of stirring, the reactor contents are filtered, and the filtrate is concentrated by evaporation under vacuum. The concentrated filtrate is then treated with sodium hydroxide solution causing the formation of the free base imino phosphoroamidite. This is extracted with dichloromethane. The extracted solution is dried with calcium chloride and the dichloromethane is removed by evaporation. The solid product is mixed with sodium hydroxide and bromoethane is charged. This results in the formation of the product tetra(diethylamino)phosphonium bromide. The product is then extracted with dichloromethane. The extract is dried and the dichloromethane is removed by evaporation. The crude product is then recrystallized from a mixture of dichloromethane and diethyl ether. The recrystallized, wet, product is then dried.
Despite the above advances in the art, it would be extremely desirable and economically advantageous if a way could be found to still further increase the efficiency of halogen exchange reactions performed using tetra(dihydrocarbylamino)phosphonium halide catalysts. It is this need to which the present invention is addressed.